A resin obtained from a polyamic acid or a polyimide precursor to which a compound having a polymerizable unsaturated linkage is chemically added or from a mixture of a polyamic acid and a compound having a polymerizable unsaturated linkage has been used as a photosensitive polyimide. A photosensitive polyimide of this kind belongs to a group of negative type photosensitive materials that form patterns by photocrosslinking of the unsaturated linkages in the presence of a photopolymerization initiator (patent documents 1 and 2). Since a polyamic acid used therein has a rigid structure, a solvent of high dissolving power such as N-methylpyrrolidone (NMP) is required as a developer after exposure to light. However, developers such as this generate a high environmental load and the disposal of waste developers incurs an enormous cost. Since an aqueous alkaline developer generates a low environmental load and is disposable at low cost, there has been a demand for photosensitive polyimides to which an aqueous alkaline developer is applicable.
Of photosensitive polyimides, the ones with a rigid backbone generally tend to have a low coefficient of linear thermal expansion. However, these photosensitive polyimides contain a large number of unsaturated linkages to act as photosensitive groups and, further, their aliphatic side chains are long; hence, they have a shortcoming that the coefficient of linear thermal expansion tends to become higher. When a photosensitive polyimide of a high coefficient of linear thermal expansion is cured, for example, on a wafer of a semiconductor, there develops a possibility that the mismatch of the coefficient of linear thermal expansion between the wafer and the polyimide after curing causes the wafer to warp. There are some indications that this warpage has a potentiality to cause inconveniences during the fabrication of semiconductors such as affecting the reliability of connection to an interposer in flip-chip package and making it difficult to handle the wafer in the process.
To solve the problem of solubility in developers among the aforementioned problems, positive type photosensitive materials are proposed wherein a polyamic acid resin to which a phenolic hydroxyl group is introduced and/or a polyamic acid resin containing a phenolic hydroxyl group such as a polybenzoxazole precursor and naphthoquinonediazide are used (patent documents 3, 4, and 5). However, in order to suppress dissolution before exposure to light and secure the alkali developing property after exposure to light, these positive type resists contain a large amount of the photosensitizer naphthoquinonediazide in relation to the resin and this is considered responsible for lowering the concentration of the resin and causing a problem in the strength of the film. Further, as a technique to overcome the problem of warpage, the incorporation of a siloxane-containing polyimide in a photosensitive resin composition is proposed to reduce the warpage by increasing the elongation of a film to be formed from the composition (patent document 6). The resin composition thus proposed surely improves the elongation, but fails to correct the mismatch of coefficient of linear thermal expansion and a strain still remains in the resin. For this reason, the proposed technique still raises concerns that such a strain might generate cracks to affect long-term reliability.
A photosensitive polyamic acid resin composition needs to be imidized generally at high temperatures above 300° C. Since a high temperature treatment such as this is difficult to perform in the manufacturing process of semiconductors, a demand has been created for photosensitive polyamic acid resin compositions that can be imidized at low temperatures. To solve this problem, the introduction of a catalyst that becomes active at the time of curing is proposed (patent documents 7 and 8). However, a matter of concern here is that the catalyst exerts adverse influences such as deterioration of storage stability and lowering of reliability by corrosion of the circuit. A photosensitive resin composition that is highly storage-stable at room temperature and requires no high-temperature treatment at the time of curing is proposed (patent document 9). However, since siloxane units are introduced into the molecule in the proposed photosensitive resin composition, a cured film obtained from the composition has a high coefficient of linear thermal expansion and is not free from the problem of warpage.